Separation apparatus incorporating at least one bleed valve is known, for example from published International Patent Application No. PCT/GB93/01325. In that arrangement, a bleed valve is arranged upstream of a cyclonic separator such that, if the pressure (which is normally directly related to the airflow) in the separator falls below a predetermined level, air is bled into the airflow path from the atmosphere in order to maintain a minimum airflow. This allows the cyclonic separator to operate satisfactorily and ensures that the motor is effectively cooled. Whilst this known arrangement is perfectly adequate in most cases, it has been found that there are some instances when its performance could be improved. For example, a vacuum cleaner will normally include a filter, separate from the main dust-separating apparatus, located either immediately upstream or downstream of the motor. If this filter becomes clogged, the airflow through the dust-separating apparatus (i.e. the cyclonic separator) will be reduced and this can prevent sufficient suction being developed in the dust-separating apparatus to cause the bleed valve to bleed air into the dust-separating apparatus. The result can be reduced effectiveness or efficiency of the cyclonic separator and, more importantly, an increased risk of the motor overheating.
It is also known from UK patent No. 1080504 to provide a signal device for a vacuum cleaner which operates in response to an increase in pressure differential across the bag or filter to indicate that the filter requires changing. The signal is given by bleeding air along a conduit and across a reed to give an audible indication that the filter should be changed.
It is an object of the present invention to provide a bleed valve which is not dependent upon absolute pressures in order to operate.
It is also an object of the present invention to provide improved apparatus for separating particles from a fluid flow, particularly suitable for use in a vacuum cleaner, which is capable of reliably bleeding air into the separation apparatus so as to maintain a minimum airflow therein and to reduce the risk of the motor overheating.
A further object of the invention is to provide improved apparatus for separating particles from a fluid flow, particularly suitable for use in a vacuum cleaner, which can be utilised to indicate that a blockage is present.